The requirement for ATT syntax was set informally in 2001, when I had some partners from U. Md. who were advocating for Intel syntax. We discovered that having two syntaxes is unworkable.
From that time we assumed that everyone would use a common syntax. It
certainly never occurred to us that we had to say it explicitly, since it seems obvious that uniform assembly syntax is a good idea, and that the syntax we currently use, and that our tools use, should determine the assembly syntax for new code.
The change to the guidelines is hence a codification of a practice that goes back to the project's beginnings.
thanks
ron